PC85232-1 Просмотр технического описания (PDF) - NXP Semiconductors.
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PC85232-1 Datasheet PDF : 54 Pages
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NXP Semiconductors
PCF85132
LCD driver for low multiplex rates
7.3 LCD voltage selector
The LCD voltage selector coordinates the multiplexing of the LCD in accordance with the
selected LCD drive configuration. The operation of the voltage selector is controlled by the
mode-set command from the command decoder. The biasing configurations that apply to
the preferred modes of operation, together with the biasing characteristics as functions of
VLCD and the resulting discrimination ratios (D) are given in Table 5.
Table 5. Biasing characteristics
LCD drive
mode
Number of:
LCD bias
Backplanes Levels configuration
static
1
1:2 multiplex 2
1:2 multiplex 2
1:3 multiplex 3
1:4 multiplex 4
2
static
3
1⁄2
4
1⁄3
4
1⁄3
4
1⁄3
V-----o--f--f--(--R---M-----S---)
VLCD
0
0.354
0.333
0.333
0.333
-V----o--n---(--R----M----S---)
VLCD
1
0.791
0.745
0.638
0.577
D = -V----o---n--(--R----M----S----)
Voff(RMS)
∞
2.236
2.236
1.915
1.732
A practical value for VLCD is determined by equating Voff(RMS) with a defined LCD
threshold voltage (Vth), typically when the LCD exhibits approximately 10 % contrast. In
the static drive mode a suitable choice is VLCD > 3Vth.
Multiplex drive modes of 1:3 and 1:4 with 1⁄2 bias are possible but the discrimination and
hence the contrast ratios are smaller.
Bias is calculated by -----1------- , where the values for a are
1+a
a = 1 for 1⁄2 bias
a = 2 for 1⁄3 bias
The RMS on-state voltage (Von(RMS)) for the LCD is calculated with Equation 1:
Von(RMS)
=
VLCD
--a---2----+-----2---a----+-----n---
n × (1 + a)2
(1)
where the values for n are
n = 1 for static drive mode
n = 2 for 1:2 multiplex drive mode
n = 3 for 1:3 multiplex drive mode
n = 4 for 1:4 multiplex drive mode
The RMS off-state voltage (Voff(RMS)) for the LCD is calculated with Equation 2:
PCF85132
Product data sheet
Voff(RMS)
=
VLCD
--a---2----–----2----a----+-----n---
n × (1 + a)2
(2)
Discrimination is the ratio of Von(RMS) to Voff(RMS) and is determined from Equation 3:
D = -V----o---n--(--R---M----S---) = (---a-----+-----1----)--2----+-----(--n-----–----1----)
Voff(RMS)
(a – 1)2 + (n – 1)
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 23 November 2010
(3)
© NXP B.V. 2010. All rights reserved.
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